Any ideas why a hard disk that passes short/acoustic, extended, and SMART tests would degrade in a RAID1 array?

A while back, my RAID1 array showed a degraded status. I was unable to see the array after booting into Windows (my boot drive is an SSD).

After unhooking the data cables from each drive (so each disk in RAID1 would run independent), I found the problem drive and have left it out of the system ever since. I just now have finally begun testing on the drive.

It is a WD Black 1TB drive (WD1001FALS; older model, almost at it's 5 year warranty mark).

I've ran WD Diagnostics on it (including a nearly 12 hour extended test), and it's passing everything. Why would it have degraded in the RAID1? Wouldn't it at least be coming up with bad sectors or something?! I'm thoroughly confused.

I was planning to RMA the damn thing, but now if I can't prove there's anything wrong with it, I guess I need to just wipe it an rebuild the array.

Have you by any change reseted the BIOS settings at some point?
I did that mistake once when i was using a Raid0 setup. There is an option called SATA mode.
If it's set as IDE, that's your problem. It has to be RAID mode to work.

Have you by any change reseted the BIOS settings at some point?
I did that mistake once when i was using a Raid0 setup. There is an option called SATA mode.
If it's set as IDE, that's your problem. It has to be RAID mode to work.

Nope. Hadn't made any changes. I would have known because I had two RAID arrays. The RAID0 array my system drives were on were completely fine (have never degraded), but the RAID1 array did show degraded.